1. Field of the Invention
The present invention relates to a circuit board, and more particularly, to a circuit board which can decrease undesired crosstalk.
2. Description of the Prior Art
In electronics, the term “crosstalk” refers to any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another.
In telecommunications or telephony, crosstalk is often distinguishable as pieces of speech or signaling tones leaking from other connections. If the connection is analog, twisted pair cabling can often be used to reduce the effects of crosstalk. Alternatively, the signals can be converted to digital form, which is much less susceptible to crosstalk.
In an integrated circuit design, crosstalk normally refers to a signal affecting another nearby signal. Usually the coupling is capacitive, and to the nearest neighbor, but other forms of coupling and effects on signals further away are sometimes important, especially in analog designs. Please refer to FIG. 1. FIG. 1 is a schematic top view of a prior art circuit board 100. The circuit board 100 includes a signal line plane 110 and a reference plane 120, where the signal line plane (e.g., a top plane as shown in FIG. 1) 110 is parallel with the reference plane (e.g., a bottom plane as shown in FIG. 1) 120 in a thickness direction of the circuit board 100. The signal line plane 110 has a first transmission line 112 and a second transmission line 114 formed thereon, wherein the first transmission line 112 and the second transmission line 114 have no intersection on the signal line plane 110. The reference plane 120 has a conductive region 122 (i.e., physical board region) and a non-conductive region 124 (i.e., a scribe line or through hole). As shown in FIG. 1, the first transmission line 112 and the second transmission line 114 overlap the conductive region 122 in a thickness direction of the circuit board 100. In general, the signal line plane 110 and the reference plane 120 is spaced by a dielectric layer; however, to more clearly show the relation between the first and second transmission lines 112, 114 on the signal line plane 110 and the conductive region 122 and the non-conductive region 124 on the reference plane 120, the dielectric layer is not shown in FIG. 1. As shown in FIG. 1, a projection of the first transmission line 112 onto the reference plane 120 in a thickness direction of the circuit board 100 and a projection of the second transmission line 114 onto the reference plane 120 in the thickness direction of the circuit board 100 have intersections with the non-conductive region 124. The substrate coupling will cause the crosstalk to be conveyed through the circuit board 100.
Therefore, how to decrease crosstalk effectively in a circuit board is an urgent issue that needs to be resolved.